The present invention can best be understood in the context of its contribution to conventional FCC processes. Accordingly, a brief discussion of conventional cracking processes and catalysts follows.
Conversion of heavy petroleum fractions to lighter products by catalytic cracking is well known in the refining industry. Fluidized Catalytic Cracking (FCC) is particularly advantageous for the purpose. The heavy feed contacts hot regenerated catalyst and is cracked to lighter products. Carbonaceous deposits form on the catalyst, thereby deactivating it. The deactivated (spent) catalyst is separated from cracked products, stripped of strippable hydrocarbons and conducted to a regenerator, where coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor. The reactor-regenerator assembly are usually maintained in heat balance. Heat generated by burning the coke in the regenerator provides sufficient thermal energy for catalytic cracking in the reactor. Control of reactor conversion is usually achieved by controlling the flow of hot regenerated catalyst to the reactor to maintain a desired reactor temperature.
In most modern FCC units, the hot regenerated catalyst is added to the feed at the base of a riser reactor. The fluidzation of the solid catalyst particles may be promoted with a lift gas. Mixing and atomization of the feedstock may be promoted with steam, equal to 1-5 wt. % of the hydrocarbon feed. Hot catalyst from the regenerator is contacted with preheated (150.degree.-375.degree. C.) charge stock. The catalyst vaporizes and heats the feed to the desired cracking temperature. During the upward passage of the catalyst and feed, the feed is cracked, and coke deposits on the catalyst. The coked catalyst and the cracked products exit the riser and enter a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked products pass to product separation. Typically, the cracked hydrocarbon products are fractionated into a series of products, including gas, gasoline, light gas oil, and heavy cycle gas oil. Some heavier than gasoline cycle oils may be recycled to the reactor. The bottoms product, a "slurry oil", may be allowed to settle. The catalyst rich solids portions of the settled product may then be recycled to the reactor. The clarified slurry oil is a heavy product.
The "reactor vessel" into which the riser discharges primarily separates catalyst from cracked products, and permits catalyst stripping.
Older FCC units use some or all dense bed cracking. Down flow operation is also possible, in which case catalyst and oil are added to the top of a vertical tube, or "downer," with cracked products removed from the bottom of the downer. Moving bed analogs of the FCC process, such as Thermofor Catalytic Cracking (TCC) are also known.
Further details of FCC processes can be found in: U.S. Pat. Nos. 3,152,065 (Sharp et al); 3,261,776 (Banman et al); 3,654,140 (Griffel et al); 3,812,029 (Snyder); 4,093,537, 4,118,337, 4,118,338, 4,218,306 (Gross et al); 4,444,722 (Owen); 4,459,203 (Beech et al); 4,639,308 (Lee); 4,675,099, 4,681,743 (Skraba) as well as in Venuto et al, Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979). The entire contents of these patents and publication are incorporated herein by reference.
Conventional FCC catalysts are usually finely divided acidic zeolites, preferably low coke-producing, high silica zeolite cracking catalysts comprising faujasite, rare earth Y (REY), dealuminized Y (DEALY), Ultrastable Y (USY), RE-USY, Ultrahydrophobic Y (UHP-Y) and other large pore zeolites.
Typically, FCC catalysts are fine particles having an average particle diameter of 20 to 100 microns, usually around 60-80 microns.
Catalyst for use in moving bed catalytic cracking units (TCC units) can be in the form of spheres, pills, beads, or extrudates, and can have a diameter ranging from 1 to 6 mm.
Although the catalytic cracking process is highly efficient, and is the preferred process in many refineries for converting heavier hydrocarbons to lighter, more valuable products, there are still some areas where improvements are needed.
Catalyst stripping is one of these areas. After the catalytic cracking reaction is completed, the catalyst must be stripped of strippable hydrocarbons before being regenerated. The goal of this stripping operation is to remove all strippable materials, leaving only coke.
In many catalytic cracking units, especially fluidized catalytic cracking units, much of the so called "coke" is actually valuable hydrocarbon product which is entrained, adsorbed, or otherwise present on the spent catalyst. The same thing is true in moving bed catalytic cracking.
Many attempts have been made to improve the efficiency of catalyst stripping. There is strong incentive for such improvements, because better stripping will reduce emissions of sulfur and nitrogen compounds from the regenerator, increase the recovery of valuable hydrocarbon product, unload the regenerator and reduce the amount of water of combustion formed in the regenerator. Each of these aspects will be discussed below.
Sulfur and nitrogen emissions are limiting factors in many refineries. By this, it is meant that the refinery is operating at the upper limit permitted by local regulations for emissions of SO.sub.x and/or NO.sub.x from the FCC flue gas. Sulfur emissions can be most easily controlled, and most expensively, by hydrotreating the feed to remove sulfur compounds. Another alternative is running only sweet crudes, with a lower sulfur content through the catalytic cracking unit. Another approach is to add an SO.sub.x acceptor material to the unit, which releases H.sub.2 S in the catalytic cracking reactor side. Yet another approach is conventional stack gas clean-up. Nitrogen oxides are a similar problem, in that they represent a stack gas pollutant. Hydrotreating, a change in FCC or TCC regenerator operation, or going to a different crude which has less nitrogen in it, are all possible methods of controlling NO.sub.x emissions. Some of these, e.g., adjusting the regenerator operation so that a more reducing atmosphere is present therein, will reduce NO.sub.x emissions but increase CO emissions.
Increased recovery of product is important because the cracked products are extremely valuable, and they should not be merely burned, as a source of fuel in the refinery. Allowing potentially strippable hydrocarbons to enter the FCC or moving bed catalyst regeneration unit is equivalent to converting a valuable fuel oil fraction into a low value coke product. This can cause "winding down" of the unit, which reduces conversion.
Water of combustion is also a severe problem in catalytic cracking regenerators, because the hydrocarbons burn to form H.sub.2 O and CO.sub.2. Much of the steam in the regenerator is the result of bad stripping. Steam partial pressures of 5-10 psia are a fact of life in many FCC regenerators. The H.sub.2 O makes the FCC or TCC regenerator a catalyst "steamer", and this steaming leads to severe and rapid hydrothermal deactivation of the high activity, zeolite based catalyst used in these processes. The average effective life of zeolite based catalyst in many FCC units is on the order of 5-15 days. Anything that can be done to reduce the steam partial pressure in the regenerator will result in a significant increase in catalyst life.
Many catalytic cracking units are constrained solely by the capacity of the regenerator to burn off "coke" and create freshly regenerated catalyst. These units could be more efficiently run, and run at higher feed rates, if less valuable hydrocarbon were being burned in the regenerator. Ideally, the catalytic cracking regenerator should remove only catalytic coke, all other forms of coke either being removed or not created in the FCC reactor. This is the goal to which all refiners aspire, and yet in most regenerators only about 1/3-178 of the material burned in the regenerator is catalytic coke. 1/2
Several attemps have been made to improve on stripping operations by increasing either stripping time or temperature. A very promising approach is that taken in U.S. No. 4,481,103, Fluidized Catalytic Cracking Process With Long Residence Time Steam Stripper, which is incorporated herein by reference. The only drawback to such an approach is that an additional vessel is required to achieve the long residence time stripping, typically 1-5 minutes. Although this approach effectively reduces the burning load on the regenerator, increases recovery of valuable products, and reduces sulfur emissions, the benefits are not quite as great as desired. There is also a slight disadvantage in that fairly long residence time of catalyst in the presence of steam is required. At the relatively low temperatures involved in U.S. No. 4,481,103, somewhat lower than the riser top temperature, there is no significant catalyst deactivation, and catalyst life is probably improved overall, rather than reduced because of the stripping operation.
Another approach is high temperature stripping. It is known to operate a stripping zone with some air or oxygen addition. The high temperature stripping, with some combustion, will be highly effective at removing potentially strippable hydrocarbons (by burning them!) and reducing the burning load in the regenerator, but there are several drawbacks. The "stripper" flue gas will be contaminated with significant amounts of nitrogen (when air is used as the oxygen containing gas), carbon monoxide, and SO.sub.x. There will be enough of these materials around that the stripper effluent can no longer be mixed with the cracked products for production of more valuable hydrocarbons.
In U.S. Pat. No. 4,820,404, which is incorporated herein by reference, a preferred approach to high temperature stripping is disclosed. The spent catalyst is mixed with some hot regenerated catalyst to form a high temperature combined catalyst stream. This high temperature mixture can be very effectively stripped with stripping steam. The stripper effluent combines, as in the prior art stripping methods, with cracked vapors and the combined hydrocarbon streams are sent to conventional product recovery facilities. The stripped catalyst is then cooled and regenerated. This approach improves the efficiency of catalyst stripping, because of the higher temperatures involved, but there are some drawbacks. Operation with a stripper container, e.g., a 50/50 mix of spent catalyst and hot regenerated catalyst requires that the stripper handle twice the catalyst flow as it did previously. There is a minor amount of hydrothermal deactivation. Some hot regenerated catalyst will see a fairly severe steaming atmosphere in the stripper. The spent catalyst will be subjected to steam stripping at a higher than normal temperature. These are minor effects. Overall this process should extend catalyst life as compared to conventional FCC units not using a hot stripper because the water precursors will be kept out of the regenerator.
A different approach, one focusing on the problem of increased SO.sub.x emissions from the FCC regenerator, is disclosed in U.S. No. 4,267,072, which is incorporated herein by reference. In this patent a metallic reactant is added to the circulating FCC catalyst inventory. The metallic reactant reacts with sulfur oxides in the regeneration zone and forms a stable metal- and sulfur containing compound. These sulfur-containing compounds are reported to break down to sulfur-containing gas which is withdrawn from the stripping zone.
We realized that none of the approaches discussed above could be completely satisfactory. The long residence time approaches were not particular for many units, which did not have the physical space to put in a long residence time stripper. High temperature stripping also requires significant unit modifications.
We realized that it was essential to make a radical departure from prior art FCC catalyst stripping procedures to achieve a significant improvement in the stripping operation.
We realized that microwave energy could be used to make catalyst stripping more efficient.
Candor compels mention of much prior work that has gone on the use of microwave energy in hydrocarbon conversion. Ever since the work reported in U.S. Pat. No. 3,503,865, which is incorporated herein by reference, researchers have known that microwave energy could be used to efficiently heat heavy, hydrocarbonaceous materials such as coal. In U.S. No. 3,503,865, coal was liquified using microwave energy.
Microwave radiation was used to enhance crystallization of zeolites in U.S. No. 4,778,666, Chu et al, which is incorporated herein by reference.
Attempts have been made to use microwave energy for in situ tar sands or heavy oil recovery projects. These uses of microwave energy have not been too successful. The material to be recovered (tar sands or heavy oil) was a relatively low value product. In-situ heating of it required that 10 tons or rock, sand, etc. has to be heated to recover 1 ton of low value material.
A much more efficient use of microwave energy for enhancement of hydrocarbon conversion processes was reported in U.S. No. 4,545,879, which is incorporated herein by reference. This patent discloses a technique for desulfurizing hydrocracked petroleum pitch containing organic molecules having chemically bound sulfur. Particles of petroleum pitch and a para- or ferro magnetic material catalyst were intimately mixed with the pitch, and the mixture subjected to microwave radiation in the presence of hydrogen to generate a high intensity oscillating electric field. This released at least part of the chemically bound sulfur from the pitch as sulfur-containing gases without substantial increase of the temperature of the pitch. Use of microwave irradiation, gated in a train of short pulses, minimized heating of the pitch.
An invitation to use microwave energy in various petroleum refinery operations is reported in U.S. No. 4,279,722, which is incorporated herein by reference. This patent suggested that the catalytic cracking operation would be improved by subjecting the feed and the cracking catalyst to microwave energy. The patentee specified that the microwave source is spaced in the riser cracker, and, " . . . where desirable, an additional source which may be of a different frequency is placed in the reactor."
U.S. No. 4,144,189, which is incorporated herein by reference, taught regenerating spent FCC catalyst in the presence of microwave energy. The spent catalyst would be fluidized with hydrogen and microwaved to convert the coke to volatile products which would be removed with the hydrogen, so that regenerated catalyst could be returned to the reactor. In an alternative embodiment, the patentee disclosed contacting the spent catalyst with a solvent then microwaving the solvent/catalyst slurry to regenerate the catalyst.
U.S. No. 4,076,607, which is incorporated herein by reference, discloses a process for coal desulfurization generating extremely low amounts of heat. The patentee taught use of microwave energy to introduce thermochemical, in-situ reactions to liberate sulfur in the form of stable gaseous species, such as H.sub.2 S, COS and SO.sub.2.
None of the prior workers in the microwave field address the problems of improving the operation of the FCC stripper in a practical manner. Most of the microwave processes make poor use of an expensive energy source. This can be better understood by considering what goes on in an FCC.
Operation of a catalytic cracking unit using microwave energy to, e.g., activate a mixture of fresh feed and catalytic cracking catalyst would require enormous amounts of microwave energy. In FCC units there are usually 3-10 weights of catalyst per weight of oil. A large FCC unit might be a 50,000 BPD unit. The amount of energy needed to microwave 50,000 BPD of oil plus perhaps 5 times the weight of this oil in hot catalyst is enormous. The improvements expected in the catalyst cracking operation are not sufficient, it is believed, to justify such an expense.
Use of microwave energy to regenerate spent catalytic cracking catalyst could result in extremely high energy cost. Quite a lot of hydrogen would be required to gasify coke back to hydrocarbons in a hydrogen fed, microwave irradiated FCC regenerator. The catalyst resulting from such a regeneration would not be an especially high temperature, so some other source of heat would be needed to supply the endothermic heat of the catalytic cracking reaction. Regeneration of FCC catalyst by contact with a solvent also would require an enormous expenditure of energy, and would not produce the hot regenerated catalyst necessary to supply the heat needed for the cracking reaction.
We realized that modern FCC units operate fairly efficiently in both the riser reactor and in the regenerator. By this we do not mean that all things are perfect, but that only minor improvements in the operation of the riser reactor or the FCC regenerator are all that can be hoped for in such a mature process. The only area in catalytic cracking where gross inefficiences remain is the catalyst stripper. FCC operators have known that 1/3-1/2 of the so called "coke" remaining on catalyst fed to the regenerator is actually the product of an inefficient stripping operation.
We have now discovered a way to profoundly improve the operation of the FCC catalyst stripper.